The present invention generally relates to deflection control type ink jet printing apparatuses and, more particularly, to a deflection detector for discriminating proper and inproper deflections and/or amounts of deflection of charged ink droplets.
In an ink jet printer of the type described, an ink is ejected under supersonic vibration from a nozzle and then separated into droplets at regular intervals at a position advanced a predetermined distance from the nozzle. Timed to the separation of ink droplets, a charging electrode selectively applies a charging electric field to the ink droplets to deposit electrostatic charges thereon. Thereafter, deflection electrodes deflect charged ink droplet passing therebetween in accordance with their charges, so that the ink droplets impinge on a sheet of paper to print out desired data.
Deflection of charged ink droplets depend on the ink pressure, ink temperature, amplitude of the supersonic vibration, a charging timing, a charging voltage, a deflecting voltage and other various factors, which obstructs easy setting of the deflections. A widespread practice is detecting a deflection of charged ink droplets and controlling the pressure or temperature of ink and the charging voltage as well as the others until the actual deflection coincides with predetermined one. In one of various deflection detecting methods heretofore proposed, a pair of spaced charge detecting electrodes are arranged adjacent to a predetermined deflection path or reference path with the center of their spacing registered with the reference path, as disclosed in Japanese Patent no. 52-47284/1977 and "IBM Journal", January 1977, pp. 52-55. Potentials induced in the electrodes are coupled to a differential amplifier so that a deflection position can be determined depending on the polarity and level of the output signal of the differential amplifier.
Another known method of similar type is designed for a higher accuracy of detection which precludes errors due to noise, as described in Japanese Patent Application no. 55-153558/1980. In this method, a charge pattern consists of a string of "m (integer)" successive ink droplets charged to one polarity and a string of "n (integer)" successive ink droplets charged to the other polarity or non-charged occurring in an alternate order. The output potentials of the two electrodes are individually rectified and coupled to a differential amplifier whose output is discriminated as two electric signals of different polarities. When a first electric signal is high or logical "1" level and a second is low or logical "0" level, ink droplets are determined to have moved along a path above the reference path (excessive deflection). When the first signal is low or logical "0" level and the second high or logical "1" level, the actual path of ink droplets is determined to be below the reference path (short deflection). If both the signals are low or logical " 0" level, the actual path is identified with the reference path.
However, a problem has existed in these prior art deflection detecting methods in that a distance ink droplets are expected to fly must be increased to accommodate the detection electrodes which are arranged along the path of ink droplets. Another problem is that a substantial period of time is required to detect a deflection position in order to enhance the stability in detection.